Compound semiconductor device having overhang-shaped gate

ABSTRACT

A compound semiconductor device includes: a compound semiconductor layer; a protective insulating film that covers a top of the compound semiconductor layer; and a gate electrode formed on the protective insulating film, wherein the protective insulating film has a first trench and a second trench which is formed side by side with the first trench and in which the protective insulating film remains with only a predetermined thickness on the compound semiconductor layer, and wherein the gate electrode fills the first trench, and one end of the gate electrode is away from the first trench and located at least in the second trench.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2012-155084, filed on Jul. 10,2012, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a compoundsemiconductor device and a method of manufacturing the same.

BACKGROUND

Semiconductor devices, in particular, nitride semiconductor devices havebeen actively developed as high-withstand-voltage, high-powersemiconductor devices, by utilizing their characteristics such as a highsaturation electron velocity, a wide band gap, and so on. Many reportshave been made on field-effect transistors, in particular, HEMTs (HighElectron Mobility Transistors) as the nitride semiconductor devices.Especially, an AlGaN/GaN HEMT using GaN as an electron transit layer andusing AlGaN as an electron supply layer has been drawing attention. Inthe AlGaN/GaN HEMT, a distortion resulting from a difference in latticeconstant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectricpolarization caused by the distortion and to spontaneous polarization ofAlGaN, a high-concentration two-dimensional electron gas (2DEG) isobtained. This makes it possible to realize high withstand voltage andhigh output power.

-   Patent Document 1: Japanese Laid-open Patent Publication No.    2003-59944-   Patent Document 2: Japanese Laid-open Patent Publication No.    2000-100831

For the HEMT, research and development are advanced for a gate electrodewhich can reduce the gate capacitance and the gate resistance in orderto improve the high-frequency characteristics. An HEMT is devised whichhas a gate electrode in a so-called overhanging shape composed of anarrow fine gate and a wide over gate thereon. In the HEMT, when a highdrain voltage is applied, a high electric field is applied around thegate electrode. In particular, very high electric fields concentrate onthe fine gate end and the over gate end. This high electric fielddamages semiconductor crystals at the fine gate end and damages aprotective insulating that covers the semiconductor surface at the overgate end. In either case, the high electric field causes deteriorationor breakdown of device characteristics, thereby significantly decreasingthe reliability of the device.

SUMMARY

An aspect of a compound semiconductor device includes: a compoundsemiconductor layer; a protective insulating film that covers a top ofthe compound semiconductor layer; and an electrode formed on theprotective insulating film or in an opening of the protective insulatingfilm, wherein the protective insulating film has a first trench and asecond trench which is formed side by side with the first trench,wherein the protective insulating film remains with only a thickness onthe compound semiconductor at a bottom of the second trench, and whereinthe electrode fills the first trench, and one end of the electrode isaway from the first trench and located at least in the second trench.

An aspect of a method of manufacturing a compound semiconductor deviceincludes: forming a protective insulating film that covers a top of acompound semiconductor layer and has a first trench and a second trenchwhich is formed side by side with the first trench, forming, on theprotective insulating film or in an opening of the protective insulatingfilm, an electrode that fills the first trench and has one end away fromthe first trench and located at least in the second trench, and whereinthe protective insulating film remains with only a thickness on thecompound semiconductor at a bottom of the second trench.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A to FIG. 1C are schematic cross-sectional views illustrating amethod of manufacturing a Schottky-type AlGaN/GaN HEMT according to afirst embodiment in order of processes;

FIG. 2A to FIG. 2C are schematic cross-sectional views, subsequent toFIG. 1A to FIG. 1C, illustrating the method of manufacturing theAlGaN/GaN HEMT according to the first embodiment in order of processes;

FIG. 3A to FIG. 3C are schematic cross-sectional views, subsequent toFIG. 2A to FIG. 2C, illustrating the method of manufacturing theAlGaN/GaN HEMT according to the first embodiment in order of processes;

FIG. 4A and FIG. 4B are a view illustrating a conventional AlGaN/GaNHEMT as a comparative example and a chart presenting the intensity of anelectric field applied to a region between a source and a drain thereof;

FIG. 5A and FIG. 5B are a view illustrating the AlGaN/GaN HEMT accordingto the first embodiment and a chart presenting the intensity of anelectric field applied to a region between a source and a drain thereof;

FIG. 6A and FIG. 6B are characteristic charts presenting results ofthree-terminal characteristics of the AlGaN/GaN HEMT according to thefirst embodiment investigated based on comparison with the comparativeexample;

FIG. 7 is a characteristic chart presenting results of ahigh-temperature current conduction test carried out on the AlGaN/GaNHEMT according to the first embodiment, based on comparison with thecomparative example;

FIG. 8A to FIG. 8C are schematic cross-sectional views illustrating mainprocesses in a method of manufacturing a Schottky-type AlGaN/GaN HEMTaccording to Modification Example 1 of the first embodiment;

FIG. 9 is a schematic cross-sectional view, subsequent to FIG. 8A toFIG. 8C, illustrating a main process in the method of manufacturing theSchottky-type AlGaN/GaN HEMT according to Modification Example 1 of thefirst embodiment;

FIG. 10A and FIG. 10B are a view illustrating the AlGaN/GaN HEMTaccording to Modification Example 1 of the first embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof;

FIG. 11A and FIG. 11B are characteristic charts presenting results ofthree-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 1 of the first embodiment investigated based oncomparison with the comparative example;

FIG. 12 is a characteristic chart presenting results of ahigh-temperature current conduction test carried out on the AlGaN/GaNHEMT according to Modification Example 1 of the first embodiment, basedon comparison with the comparative example;

FIG. 13A to FIG. 13C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a Schottky-type AlGaN/GaNHEMT according to Modification Example 2 of the first embodiment;

FIG. 14A and FIG. 14B are a view illustrating the AlGaN/GaN HEMTaccording to Modification Example 2 of the first embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof;

FIG. 15A and FIG. 15B are characteristic charts presenting results ofthree-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 2 of the first embodiment investigated based oncomparison with the comparative example;

FIG. 16 is a characteristic chart presenting results of ahigh-temperature current conduction test carried out on the AlGaN/GaNHEMT according to Modification Example 2 of the first embodiment, basedon comparison with the comparative example;

FIG. 17A to FIG. 17C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a Schottky-type AlGaN/GaNHEMT according to Modification Example 3 of the first embodiment;

FIG. 18 is a schematic cross-sectional view, subsequent to FIG. 17A toFIG. 17C, illustrating a main process in the method of manufacturing theSchottky-type AlGaN/GaN HEMT according to Modification Example 3 of thefirst embodiment;

FIG. 19A and FIG. 19B are a view illustrating the AlGaN/GaN HEMTaccording to Modification Example 3 of the first embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof;

FIG. 20A and FIG. 20B are characteristic charts presenting results ofthree-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 3 of the first embodiment investigated based oncomparison with the comparative example;

FIG. 21 is a characteristic chart presenting results of ahigh-temperature current conduction test carried out on the AlGaN/GaNHEMT according to Modification Example 3 of the first embodiment, basedon comparison with the comparative example;

FIG. 22A to FIG. 22C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to a second embodiment;

FIG. 23A to FIG. 23C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 1 of the second embodiment;

FIG. 24A to FIG. 24C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 2 of the second embodiment;

FIG. 25A to FIG. 25C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 3 of the second embodiment;

FIG. 26A to FIG. 26C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to a third embodiment;

FIG. 27A to FIG. 27C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 1 of the third embodiment;

FIG. 28A to FIG. 28C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 2 of the third embodiment;

FIG. 29A to FIG. 29C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 3 of the second embodiment;

FIG. 30 is a connection diagram illustrating a schematic configurationof a power supply device according to a fourth embodiment; and

FIG. 31 is a connection diagram illustrating a schematic configurationof a high-frequency amplifier according to a fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference tothe drawings. In the following embodiments, a structure of a compoundsemiconductor device will be described along with a method ofmanufacturing the compound semiconductor device.

Note that, in the following drawings, some constituent members are notillustrated with relatively accurate size and thickness for convenienceof illustration.

First Embodiment

In this embodiment, a Schottky-type AlGaN/GaN HEMT is disclosed as thecompound semiconductor device.

FIG. 1A to FIG. 1C to FIG. 3A to FIG. 3C are schematic cross-sectionalviews illustrating a method of manufacturing the Schottky-type AlGaN/GaNHEMT according to the first embodiment in order of processes.

First, as illustrated in FIG. 1A, a compound semiconductor layer 2having a stacked structure of compound semiconductors is formed on, forexample, a semi-insulating SiC substrate 1 being a growth substrate.

As the growth substrate, a Si substrate, a sapphire substrate, a GaAssubstrate, a GaN substrate, or the like may be used instead of the SiCsubstrate. The conductivity of the substrate may be eithersemi-insulating or conductive.

The compound semiconductor layer 2 includes a buffer layer 2 a, anelectron transit layer 2 b, an intermediate layer 2 c, an electronsupply layer 2 d and a cap layer 2 e. In the AlGaN/GaN HEMT,two-dimensional electron gas (2DEG) is generated in the vicinity of aninterface, of the electron transit layer 2 b, with the electron supplylayer 2 d (to be exact, the intermediate layer 2 c).

More specifically, on the SiC substrate 1, the following compoundsemiconductors are grown by, for example, an MOVPE (Metal Organic VaporPhase Epitaxy) method. An MBE (Molecular Beam Epitaxy) method or thelike may be used instead of the MOVPE method.

On the SIC substrate 1, AlN, i (intentionally undoped)-GaN, i-AlGaN,n-AlGaN and n-GaN are sequentially deposited to stack and form thebuffer layer 2 a, the electron transit layer 2 b, the intermediate layer2 c, the electron supply layer 2 d and the cap layer 2 e. As the growthcondition of AlN, GaN, AlGaN and GaN, a mixed gas of trimethylaluminumgas, trimethylgallium gas and ammonia gas is used as a source gas.Depending on the compound semiconductor layer that is to be grown,whether or not to supply the trimethylaluminum gas that is an Al sourceand the trimethylgallium gas that is a Ga source and their flow ratesare appropriately set. A flow rate of the ammonia gas being a commonsource is set to about 100 sccm to about 10 LM. Further, growth pressureis set to about 50 Torr to about 300 Torr, and growth temperature is setto about 1000° C. to about 1200° C.

To grow GaN and AlGaN as an n-type, for example, SiH₄ gas containing Siis added as n-type impurity to the source gas at a predetermined flowrate, thereby doping GaN and AlGaN with Si. A doping concentration of Siis set to about 1×10¹⁸/cm³ to about 1×10²⁰/cm³, for example, set toabout 5×10¹⁸/cm³.

Here, the buffer layer 2 a is formed with a thickness of about 0.1 μm,the electron transit layer 2 b is formed with a thickness of about 3 μm,the intermediate layer 2 c is formed with a thickness of about 5 nm, theelectron supply layer 2 d is formed with a thickness of about 20 nm andan Al ratio of about 0.2 to about 0.3, and the cap layer 2 e is formedwith a thickness of about 10 nm.

Subsequently, element isolation structures 3 are formed as illustratedin FIG. 1B.

More specifically, argon (Ar), for instance, is injected to elementisolation regions of the compound semiconductor layer 2. Thus, theelement isolation structures 3 are formed in the compound semiconductorlayer 2 and in a surface layer portion of the SiC substrate 1. Theelement isolation structures 3 demarcate an active region on thecompound semiconductor layer 2.

Incidentally, instead of the above injection method, an STI (ShallowTrench Isolation) method, for instance, may be performed for the elementisolation.

Subsequently, as illustrated in FIG. 1C, a source electrode 4 and adrain electrode 5 are formed.

More specifically, electrode trenches 2A, 2B are first formed in the caplayer 2 e at formation scheduled positions for a source electrode and adrain electrode in a surface of the compound semiconductor layer 2.

A resist mask having openings at the formation scheduled positions forthe source electrode and the drain electrode in the surface of thecompound semiconductor layer 2 is formed. By using this resist mask, thecap layer 2 e is removed by dry etching. Thus, the electrode trenches2A, 2B are formed. An inert gas such as Ar and chlorine gas such as Cl₂are used as an etching gas for the dry etching. Here, the electrodetrenches may be formed by dry etching to penetrate through the cap layer2 e down to a surface layer portion of the electron supply layer 2 d.

As an electrode material, Ti/Al are used, for instance. To form theelectrodes, an eaves-structure two-layer resist suitable for a vapordeposition method and a liftoff method is used. This resist is appliedon the compound semiconductor layer 2 to form a resist mask havingopenings at the electrode grooves 2A, 2B. Ti/Al are deposited by usingthis resist mask. A thickness of Ti is about 20 nm and a thickness of Alis about 200 nm. By the liftoff method, the resist mask with the eavesstructure and Ti/Al deposited thereon are removed. Thereafter, the SiCsubstrate 1 is heat-treated at about 550° C. in, for example, a nitrogenatmosphere, and the residual Ti/Al are brought into ohmic contact withthe electron supply layer 2 d. Through the above processes, the sourceelectrode 4 and the drain electrode 5 having the electrode trenches 2A,2B embedded under Ti/Al are formed.

Subsequently, as illustrated in FIG. 2A, a protective insulating film 6is formed.

More specifically, an insulator, for example, silicon nitride (SiN) isdeposited on the whole surface of the compound semiconductor layer 2 to,for example, a thickness of about 60 nm by a plasma CVD method or thelike. Thus, the protective insulating film 6 is formed.

As the material of the protective insulating film, alumina (Al₂O₃),silicon oxide (SiO₂), silicon oxynitride (SiON) or the like can be usedinstead of SiN.

Subsequently, as illustrated in FIG. 2B, a second trench 6 b is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 400 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 11having an opening 11 a is formed.

Next, dry etching using the resist mask 11 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness at the bottom of the opening11 a. For example, SF₆ is used as an etching gas. Thus, the secondtrench 6 b having a width of about 400 nm and a depth of, for example,about 30 nm (the thickness of the remaining protective insulating film 6is about 30 nm) is formed in the protective insulating film 6. Thesecond trench 6 b is formed at a site biased toward the drain electrode5, here, a site where 0.2 μm or more of an over gate of a gate electrodeto be formed is contained in the trench. A correct value of the depth ofthe second trench 6 b is decided depending on a thickness of theprotective insulating film 6, a dielectric breakdown withstand voltageof the protective insulating film 6, a potential difference between adrain voltage and a gate voltage, a peak value of swing of the gatevoltage and soon.

The resist mask 11 is removed by ashing using oxygen plasma or wettreatment using a chemical.

Subsequently, as illustrated in FIG. 2C, a first trench 6 a is formed inthe protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 12having an opening 12 a is formed.

Next, dry etching using the resist mask 12 is performed on theprotective insulating film 6 until the surface of the cap layer 2 e isexposed at the bottom of the opening 12 a. For example, SF₆ is used asan etching gas. Thus, the first trench 6 a that is a through trenchhaving a width of about 600 nm and exposing the surface of the cap layer2 e is formed in the protective insulating film 6. The first trench 6 ais formed at a formation scheduled site for a fine gate of a gateelectrode to be formed at subsequent processes, side by side with thesecond trench 6 b in the protective insulating film 6.

The resist mask 12 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 a after forming the second trench6 b in the protective insulating film 6 is exemplified in FIG. 2B andFIG. 2C, but the order of processes may be inverted so that the secondtrench 6 b may be formed after the first trench 6 a is formed in theprotective insulating film 6.

Subsequently, as illustrated in FIG. 3A, a resist mask 13 for forming agate is formed.

More specifically, each of a lower-layer resist 13A (for example, PMGI(trade name): manufactured by Micro-Chem. Inc. in the United States) andan upper-layer resist 13B (PFI-32 (trade name): manufactured by SumitomoChemical Co., Ltd.) is first applied on the whole surface, for example,by a spin coating method. Ultraviolet exposure is performed to form anopening 13Ba, for example, having a diameter of about 1.5 μm in theupper-layer resist 13B. Next, a wet etching using an alkali developingsolution is performed on the lower-layer resist 13A while using theupper-layer resist 13B as a mask to thereby form an opening 13Aa in thelower-layer resist 13A. Thus, the resist mask 13 is formed which iscomposed of the lower-layer resist 13A having the opening 13Aa and theupper-layer resist 13B having the opening 13Ba. In the resist mask 13,an opening where the opening 13Aa and the opening 13Ba communicate witheach other is denoted by 13 a.

Subsequently, as illustrated in FIG. 3B, a gate electrode 7 is formed.

More specifically, gate metals (Ni: a thickness of about 10 nm/Au: athickness of about 300 nm) are deposited on the whole surface includingthe inside of the opening 13 a using the resist mask 13. Thus, the gateelectrode 7 is formed.

Subsequently, as illustrated in FIG. 3C, the resist mask 13 is removed.

More specifically, the SiC substrate 1 is immersed inN-methyl-pyrrolidinone warmed at 80° C., and the resist mask 13 andunnecessary gate metals are removed by the liftoff method.

The gate electrode 7 is in a so-called overhanging shape in which a finegate 7A at a lower part fills the inside of the first trench 6 a and isin Schottky contact with the surface of the compound semiconductor layer2, and an over gate 7B at an upper part is formed wider than the finegate 7A. In the gate electrode 7, one end (an electrode end on the drainelectrode 5 side, defined as an OG end 7Ba) of the over gate 7B islocated inside the second trench 6 b. Specifically, the OG end 7Ba isformed at a site, inside the second trench 6 b, away from an end portionon the gate electrode 7 side of the second trench 6 b toward the drainelectrode 5 by 0.2 μm or more. An electrode end on the drain electrode 5side of the fine gate 7A is an FG end 7Aa.

Thereafter, through processes of electrical connection of the sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the Schottky-type AlGaN/GaN HEMT is formed.

Hereinafter, operations and effects that the AlGaN/GaN HEMT according tothis embodiment has will be described based on comparison with acomparative example.

FIG. 4A and FIG. 4B are a view illustrating a conventional AlGaN/GaNHEMT as the comparative example of this embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof. FIG. 4A is a schematiccross-sectional view of the AlGaN/GaN HEMT and FIG. 4B presents acharacteristic chart of the electric field intensity. FIG. 5A and FIG.5B are a view illustrating the AlGaN/GaN HEMT according to thisembodiment and a chart presenting the intensity of an electric fieldapplied to a region between a source and a drain thereof. FIG. 5A is aschematic cross-sectional view of the AlGaN/GaN HEMT corresponding toFIG. 3C and FIG. 5B presents a characteristic chart of the electricfield intensity.

In the Schottky-type AlGaN/GaN HEMT of the comparative example, aprotective insulating film 101 is formed as illustrated in FIG. 4Ainstead of the protective insulating film 6 in FIG. 3C, and a gateelectrode 102 is formed instead of the gate electrode 7. The protectiveinsulating film 101 is formed to be thinner than the protectiveinsulating film 6 and have a thickness of, for example, about 50 nm. Inthe protective insulating film 101, an opening 101 a that is a throughtrench corresponding to the first trench 6 a in the protectiveinsulating film 6 is formed but a trench corresponding to the secondtrench 6 b is not formed. The gate electrode 102 is formed in anoverhanging shape in which a narrow fine gate 7A fills the opening 101 aand is in Schottky contact with a surface of a compound semiconductorlayer 2 and a wide over gate 7B are integrated.

FIG. 4B and FIG. 5B present the electric field intensities betweenbroken lines A and broken lines B drawn in FIG. 4A and FIG. 5A, and alsopresent electric field intensities of breakdown limits of devicecharacteristics at the FG ends and OG ends due to electric fieldconcentration. The electric field intensity of breakdown limit at the FGend is defined as BE1, and the electric field intensity of breakdownlimit at the OG end is defined as BE2.

In the HEMT having the gate electrode in the overhanging shape, highelectric fields concentrate on the FG end and the OG end. In this case,the device characteristics are likely to deteriorate or break down moreat the FG end than at the OG end, so that BE1 is lower than BE2.

In the AlGaN/GaN HEMT in the comparative example, as illustrated in FIG.4B, the intensity of the electric field applied to the OG end is lowerthan the electric field intensity BE2 of the breakdown limit at the OGend and has a considerable margin with respect to BE2. In contrast, theintensity of the electric field applied to the FG end is substantiallyequal to the electric field intensity BE1 of the breakdown limit at theFG end and has little or no margin with respect to BE1.

A possible reason of the above in the comparative example is as follows.At the OG end, the over gate 7B is in contact with the protectiveinsulating film 101. Therefore, BE2 is determined by the breakdown limitof the protective insulating film 101. On the other hand, at the FG end,the fine gate 7A is in contact with the compound semiconductor layer 2and the protective insulating film 101. The semiconductor crystals inthe compound semiconductor layer 2 are much lower in breakdown limitwith respect to the electric field than the insulator of the protectiveinsulating film 101. Therefore, BE 1 is determined by the breakdownlimit of the compound semiconductor layer 2 that is lower than BE2. Asdescribed above, the protective insulating film 101 is high in breakdownlimit with respect to the electric field and relatively has a marginwith respect to the breakdown limit even if the electric fieldconcentrates on the OG end, whereas the compound semiconductor layer 2is low in breakdown limit with respect to the electric field and highlypossibly reaches the breakdown limit if the electric field concentrateson the FG end.

In the case of applying a predetermined drain voltage, the total amountof electric field generated around the gate electrode takes an almostconstant predetermined value. As described above, the deterioration orbreakdown of the device characteristics due to electric fieldconcentration most possibly occurs at the FG end, whereas there is amargin with respect to BE2 that is the breakdown limit regarding theelectric field concentration on the OG end. In this embodiment, focusingattention on this point, the electric field intensity at the OG end isaggressively increased to a limit not reaching the breakdown limit torelax the electric field concentration on the FG end by the increase. Byrelaxing the electric field concentration on the FG end which mostpossibly reaches the breakdown limit, thereby suppressing thedeterioration or breakdown of the device characteristics due to theelectric field concentration as a whole.

The above is in a close relationship also with the thickness of theprotective insulating film. With a thicker protective insulating film,the electric field concentration on the OG end is relaxed more todecrease the intensity of the electric field applied to the OG end.Along with this, the intensity of the electric field applied to the FGend increases by the decrease to result in an increase in thepossibility of reaching the breakdown limit at the FG end. In order tomore surely protect the compound semiconductor layer or to reduce thecoupling capacitance between the gate electrode and the compoundsemiconductor layer so as to cope with high-frequency, the protectiveinsulating film is required to be formed thick. Since the possibility ofreaching the breakdown limit at the FG end increases when the protectiveinsulating film is formed thick, the superiority of applying thisembodiment to relax the electric field concentration on the FG end ismore pronounced.

In the AlGaN/GaN HEMT according to this embodiment, the second trench 6b is formed in the protective insulating film 6 so as to thin theprotective insulating film 6 in the second trench 6 b. The gateelectrode 7 is formed so that the OG end 7Ba is located at the site ofthe second trench 6 b where the protective insulating film 6 is thin.This promotes extension of a depletion layer in the compoundsemiconductor layer 2. As illustrated in FIG. 5B, the electric fieldintensity at the OG end 7Ba increases to a limit not reaching BE2 thatis the breakdown limit, and the electric field intensity at the FG end7Aa decreases by the increase to relax the electric field concentration.Thus, the electric field intensity at the FG end 7Aa becomes greatlylower than BE1 that is the breakdown limit. As described above, thedeterioration or breakdown of the device characteristics due to theelectric field concentration is suppressed as a whole between the gateand the drain.

Three-terminal characteristics of the AlGaN/GaN HEMT according to thisembodiment were investigated based on comparison with the abovecomparative example. The results are presented in FIG. 6A and FIG. 6B.FIG. 6A presents the result of the comparative example, and FIG. 6Bpresents the result of this embodiment. Here, solid lines indicate IVcharacteristics at application of Vds=20 V, and broken lines indicate IVcharacteristics at application of Vds=50 V.

In FIG. 6B, improvement in current collapse was confirmed as compared toFIG. 6A. This means that the electric field concentration on the FG endwas relaxed to suppress electron capture by an electron trap.

Further, a high-temperature current conduction test was carried out onthe AlGaN/GaN HEMT according to this embodiment based on comparison withthe above comparative example. The results are presented in FIG. 7.

It was confirmed that, in this embodiment, the gate current less changedin the high-temperature current conduction test and no breakdownoccurred unlike the comparative example. In other words, application ofthe protective insulating film 6 and the gate electrode 7 in thisembodiment realizes a highly reliable AlGaN/GaN HEMT with excellentoutput characteristics.

As described above, according to this embodiment, a highly reliableAlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower.

MODIFICATION EXAMPLES

Hereinafter, modification examples of the Schottky-type AlGaN/GaN HEMTaccording to the first embodiment will be described.

Modification Example 1

Modification Example 1 is different from the first embodiment in that asecond trench to be formed in a protective insulating film is different.Note that the same constituent members and so on as those of theAlGaN/GaN HEMT according to the first embodiment will be denoted by thesame reference signs, and a detailed description thereof will beomitted.

FIG. 8A to FIG. 8C and FIG. 9 are schematic cross-sectional viewsillustrating main processes in a method of manufacturing a Schottky-typeAlGaN/GaN HEMT according to Modification Example 1 of the firstembodiment.

First, through the processes in FIG. 1A to FIG. 2A of the firstembodiment, a protective insulating film 6 that covers the top of acompound semiconductor layer 2 is formed. The appearance in this eventis illustrated in FIG. 8A.

Subsequently, as illustrated in FIG. 8B, a second trench 6 c is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 400 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 14having an opening 14 a is formed.

Next, dry etching using the resist mask 14 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness in the opening 14 a. Forexample, SF₆ is used as an etching gas. Thus, the second trench 6 chaving a width of about 400 nm and a depth of, for example, about 30 nm(the thickness of the remaining protective insulating film 6 is about 30nm) is formed in the protective insulating film 6. The second trench 6 cis formed at a site biased toward a drain electrode 5, here, a sitewhere the whole trench is contained in an over gate of a gate electrodeto be formed. A correct value of the depth of the second trench 6 c isdecided depending on a thickness of the protective insulating film 6, adielectric breakdown withstand voltage of the protective insulating film6, a potential difference between a drain voltage and a gate voltage, apeak value of swing of the gate voltage and so on.

The resist mask 14 is removed by ashing using oxygen plasma or wettreatment using a chemical.

Subsequently, as illustrated in FIG. 8C, a first trench 6 a is formed inthe protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 12having an opening 12 a is formed.

Next, dry etching using the resist mask 12 is performed on theprotective insulating film 6 until the surface of a cap layer 2 e isexposed at the bottom of the opening 12 a. For example, SF₆ is used asan etching gas. Thus, the first trench 6 a that is a through trenchhaving a width of about 600 nm and exposing the surface of the cap layer2 e is formed in the protective insulating film 6. The first trench 6 ais formed at a formation scheduled site for a fine gate of a gateelectrode to be formed at subsequent processes, side by side with thesecond trench 6 c in the protective insulating film 6.

The resist mask 12 is removed by aching using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 a after forming the second trench6 c in the protective insulating film 6 is exemplified in FIG. 8B andFIG. 8C, but the order of processes may be inverted so that the secondtrench 6 c may be formed after the first trench 6 a is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 9.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 a andis in Schottky contact with the surface of the compound semiconductorlayer 2. The over gate 7B fills the inside of the second trench 6 c andhas an OG end 7Ba located at a site away from an end portion on a drainelectrode 5 side of the second trench 6 c toward the drain electrode 5by about 0.1 g m.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the Schottky-type AlGaN/GaN HEMT is formed.

Hereinafter, operations and effects that the AlGaN/GaN HEMT according toModification Example 1 has will be described based on comparison with acomparative example.

FIG. 10A and FIG. 10B are a view illustrating the AlGaN/GaN HEMTaccording to Modification Example 1 of this embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof. FIG. 10A is a schematiccross-sectional view of the AlGaN/GaN HEMT corresponding to FIG. 9, andFIG. 10B presents a characteristic chart of the electric fieldintensity. Note that the AlGaN/GaN HEMT in the comparative example isthe same as that in FIG. 4A, and its characteristic chart of theelectric field intensity is the same as that in FIG. 4B.

FIG. 10B presents the electric field intensity between a broken line Aand a broken line B drawn in FIG. 10A, and also presents electric fieldintensities of breakdown limits of device characteristics at an FG endand an OG end due to electric field concentration. The electric fieldintensity of breakdown limit at the FG end is defined as BE1, and theelectric field intensity of breakdown limit at the OG end is defined asBE2.

In the HEMT having the gate electrode in the overhanging shape, highelectric fields concentrate on the FG end and the OG end. In this case,the device characteristics are likely to deteriorate or break down moreat the FG end than at the OG end, so that BE1 is lower than BE2.

In the case of applying a predetermined drain voltage, the total amountof electric field generated around the gate electrode takes an almostconstant predetermined value. As described above, the deterioration orbreakdown of the device characteristics due to electric fieldconcentration most possibly occurs at the FG end. In contrast, anelectric field intensity close to the breakdown limit is not found in aregion between the FG end and the OG end. In Modification Example 1,focusing attention on this point, the electric field intensity in theregion between the FG end and the OG end is aggressively increased to alimit not reaching the breakdown limit to relax the electric fieldconcentration on the FG end by the increase. In other words, a part ofthe electric field intensity at the FG end is distributed to the regionbetween the FG end and the OG end. This relaxes the electric fieldconcentration on the FG end which most possibly reaches the breakdownlimit, thereby suppressing the deterioration or breakdown of the devicecharacteristics due to the electric field concentration as a whole.

In the AlGaN/GaN HEMT according to Modification Example 1, the secondtrench 6 c is formed in the protective insulating film 6 so as to thinthe protective insulating film 6 in the second trench 6 c. The gateelectrode 7 is formed so that the over gate 7B fills and contains thesecond trench 6 c where the protective insulating film 6 is thin. Thispromotes extension of a depletion layer in the compound semiconductorlayer 2. As illustrated in FIG. 10B, the electric field intensity in theregion between the FG end 7Aa and the OG end 7Ba increases to a limitnot reaching the breakdown limit and, along with this, the peak of theelectric field intensity at the FG end 7Aa decreases to relax theelectric field concentration. Thus, the electric field intensity at theFG end 7Aa becomes greatly lower than BE1 that is the breakdown limit.In Modification Example 1, since a contributory portion of the increasein the electric field intensity in the region between the FG end 7Aa andthe OG end 7Ba is relatively large, the peak of the electric fieldintensity becomes lower than that in the comparative example also at theOG end 7Ba to relax the electric field concentration. As describedabove, the deterioration or breakdown of the device characteristics dueto the electric field concentration is suppressed as a whole between thegate and the drain.

Three-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 1 were investigated based on comparison with theabove comparative example. The results are presented in FIG. 11A andFIG. 11B. FIG. 11A presents the result of the comparative example, andFIG. 11B presents the result of Modification Example 1. Here, solidlines indicate IV characteristics at application of Vds=20 V, and brokenlines indicate IV characteristics at application of Vds=50 V.

In FIG. 11B, improvement in current collapse was confirmed as comparedto FIG. 11A. This means that the electric field concentration on the FGend was relaxed to suppress electron capture into an electron trap inModification Example 1.

Further, a high-temperature current conduction test was carried out onthe AlGaN/GaN HEMT according to Modification Example 1 based oncomparison with the above comparative example. The results are presentedin FIG. 12.

It was confirmed that, in Modification Example 1, the gate current lesschanged in the high-temperature current conduction test and no breakdownoccurred unlike the comparative example. In other words, application ofthe protective insulating film 6 and the gate electrode 7 inModification Example 1 realizes a highly reliable AlGaN/GaN HEMT withexcellent output characteristics.

As described above, according to Modification Example 1, a highlyreliable AlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower.

Modification Example 2

Modification Example 2 is different from the first embodiment in thatthe shape of a part of a protective insulating film is different. Notethat the same constituent members and so on as those of the AlGaN/GaNHEMT according to the first embodiment will be denoted by the samereference signs, and a detailed description thereof will be omitted.

FIG. 13A to FIG. 13C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a Schottky-type AlGaN/GaNHEMT according to Modification Example 2 of the first embodiment.

First, through the processes in FIG. 1A to FIG. 2B of the firstembodiment, a second trench 6 b is formed in a protective insulatingfilm 6 that covers the top of a compound semiconductor layer 2. Theappearance in this event is illustrated in FIG. 13A.

Subsequently, as illustrated in FIG. 13B, a first trench 6 d is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 15having an opening 15 a is formed.

Next, wet etching using the resist mask 15 is performed on theprotective insulating film 6 until the surface of a cap layer 2 e isexposed at the bottom of the opening 15 a. For example, bufferedhydrofluoric acid is used as an etchant. Thus, the first trench 6 d thatis a through trench exposing the surface of the cap layer 2 e is formedin the protective insulating film 6. The first trench 6 d is formed suchthat its side wall surface is formed into an inclined surface by the wetetching, the width of a bottom portion is about 600 nm, and an upperportion is wider than the bottom portion. With the first trench 6 d, theprotective insulating film 6 progressively decreases in thickness fromthe second trench 6 b toward the first trench 6 d at a site between thefirst trench 6 d and the second trench 6 b. The first trench 6 d isformed at a formation scheduled site for a fine gate of a gate electrodeto be formed at subsequent processes, side by side with the secondtrench 6 b in the protective insulating film 6.

The resist mask 15 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 d after forming the second trench6 b in the protective insulating film 6 is exemplified in FIG. 2B andFIG. 13B, but the order of processes may be inverted so that the secondtrench 6 b may be formed after the first trench 6 d is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 13C.

A gate electrode 7 is in a so-called overhanging shape in which a finegate 7A at a lower part fills the inside of the first trench 6 d and isin Schottky contact with the surface of the compound semiconductor layer2, and an over gate 7B at an upper part is formed wider than the finegate 7A. In the gate electrode 7, one end (an electrode end on a drainelectrode 5 side, defined as an OG end 7Ba) of the over gate 7B islocated inside the second trench 6 b. Specifically, the OG end 7Ba isformed at a site, inside the second trench 6 b, away from an end portionon the gate electrode 7 side of the second trench 6 b toward the drainelectrode 5 by 0.2 μm or more, here about 0.2 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the Schottky-type AlGaN/GaN HEMT is formed.

Hereinafter, operations and effects that the AlGaN/GaN HEMT according toModification Example 2 has will be described based on comparison with acomparative example.

FIG. 14A and FIG. 14B are a view illustrating the AlGaN/GaN HEMTaccording to Modification Example 2 of this embodiment and a chartpresenting the intensity of an electric field applied to a regionbetween a source and a drain thereof. FIG. 14A is a schematiccross-sectional view of the AlGaN/GaN HEMT corresponding to FIG. 13C andFIG. 14B presents a characteristic chart of the electric fieldintensity. Note that the AlGaN/GaN HEMT in the comparative example isthe same as that in FIG. 4A, and its characteristic chart of theelectric field intensity is the same as that in FIG. 4B.

FIG. 14B presents the electric field intensity between a broken line Aand a broken line B drawn in FIG. 14A, and also presents electric fieldintensities of breakdown limits of device characteristics at the FG endand the OG end due to electric field concentration. The electric fieldintensity of breakdown limit at the FG end is defined as BE1, and theelectric field intensity of breakdown limit at the OG end is defined asBE2.

In the HEMT having the gate electrode in the overhanging shape, highelectric fields concentrate on the FG end and the OG end. In this case,the device characteristics are likely to deteriorate or break down moreat the FG end than at the OG end, so that BE1 is lower than BE2.

In the case of applying a predetermined drain voltage, the total amountof electric field generated around the gate electrode takes an almostconstant predetermined value. As described above, the deterioration orbreakdown of the device characteristics due to electric fieldconcentration most possibly occurs at the FG end. In contrast, there isa margin with respect to the breakdown limit regarding the electricfield concentration on the OG end. Further, an electric field intensityclose to the breakdown limit is not found in a region between the FG endand the OG end. In Modification Example 2, focusing attention on thispoint, the electric field intensity at the OG end is aggressivelyincreased to a limit not reaching the breakdown limit and the electricfield intensity in the region between the FG end and the OG end isgradually increased to a limit not reaching the breakdown limit to relaxthe electric field concentration on the FG end by the increases. Inother words, a part of the electric field intensity at the FG end isdistributed to the OG end and the region between the FG end and the OGend. This relaxes the electric field concentration on the FG end whichmost possibly reaches the breakdown limit, thereby suppressing thedeterioration or breakdown of the device characteristics due to theelectric field concentration as a whole.

In the AlGaN/GaN HEMT according to Modification Example 2, the secondtrench 6 b is formed in the protective insulating film 6, and theinclined surface is formed between the first trench 6 d being a sitewhere the fine gate 7A is to be formed and the second trench 6 b. Thus,the protective insulating film 6 is reduced in thickness in the secondtrench 6 b and progressively reduced in thickness from the second trench6 b toward the first trench 6 d, so that a depletion layer graduallyextends inside the compound semiconductor layer 2. As illustrated inFIG. 14B, the electric field intensity at the OG end 7Ba increases to alimit not reaching the breakdown limit and the electric field intensityin the region between the FG end 7Aa and the OG end 7Ba increases to alimit not reaching the breakdown limit and, along with this, theelectric field intensity at the FG end 7Aa decreases to relax theelectric field concentration. Thus, the electric field intensity at theFG end 7Aa becomes greatly lower than BE1 that is the breakdown limit.In Modification Example 2, since a contributory portion of the increasein the electric field intensity in the region between the FG end 7Aa andthe OG end 7Ba is relatively large, the increase amount in the electricfield intensity at the OG end 7Ba is lower than that in the firstembodiment. As described above, the deterioration or breakdown of thedevice characteristics due to the electric field concentration issuppressed as a whole between the gate and the drain.

Three-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 2 were investigated based on comparison with theabove comparative example. The results are presented in FIG. 15A andFIG. 15B. FIG. 15A presents the result of the comparative example, andFIG. 15B presents the result of Modification Example 2. Here, solidlines indicate IV characteristics at application of Vds=20 V, and brokenlines indicate IV characteristics at application of Vds=50 V.

In FIG. 15B, improvement in current collapse was confirmed as comparedto FIG. 15A. This means that the electric field concentration on the FGend was relaxed to suppress electron capture into an electron trap inModification Example 2.

Further, a high-temperature current conduction test was carried out onthe AlGaN/GaN HEMT according to Modification Example 2 based oncomparison with the above comparative example. The results are presentedin FIG. 16.

It was confirmed that, in Modification Example 2, the gate current lesschanged in the high-temperature current conduction test and no breakdownoccurred unlike the comparative example. In other words, application ofthe protective insulating film 6 and the gate electrode 7 inModification Example 2 realizes a highly reliable AlGaN/GaN HEMT withexcellent output characteristics.

As described above, according to Modification Example 2, a highlyreliable AlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower.

Modification Example 3

Modification Example 3 is different from the first embodiment in that asecond trench formed in a protective insulating film and the shape of apart of the protective insulating film are different. Note that the sameconstituent members and so on as those of the AlGaN/GaN HEMT accordingto the first embodiment will be denoted by the same reference signs, anda detailed description thereof will be omitted.

FIG. 17A to FIG. 17C and FIG. 18 are schematic cross-sectional viewsillustrating main processes in a method of manufacturing a Schottky-typeAlGaN/GaN HEMT according to Modification Example 3 of the firstembodiment.

First, through the processes in FIG. 1A to FIG. 2A of the firstembodiment, a protective insulating film 6 that covers the top of acompound semiconductor layer 2 is formed. The appearance in this eventis illustrated in FIG. 17A.

Subsequently, as illustrated in FIG. 17B, a second trench 6 c is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 400 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 14having an opening 14 a is formed.

Next, dry etching using the resist mask 14 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness in the opening 14 a.

For example, SF₆ is used as an etching gas. Thus, the second trench 6 chaving a width of about 400 nm and a depth of, for example, about 30 nm(the thickness of the remaining protective insulating film 6 is about 30nm) is formed in the protective insulating film 6. The second trench 6 cis formed at a site biased toward a drain electrode 5, here, a sitewhere the whole trench is contained in an over gate of a gate electrodeto be formed. A correct value of the depth of the second trench 6 c isdecided depending on a thickness of the protective insulating film 6, adielectric breakdown withstand voltage of the protective insulating film6, a potential difference between a drain voltage and a gate voltage, apeak value of swing of the gate voltage and so on.

The resist mask 14 is removed by ashing using oxygen plasma or wettreatment using a chemical.

Subsequently, as illustrated in FIG. 17C, a first trench 6 d is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 15having an opening 15 a is formed.

Next, wet etching using the resist mask 15 is performed on theprotective insulating film 6 until the surface of a cap layer 2 e isexposed at the bottom of the opening 15 a. For example, bufferedhydrofluoric acid is used as an etchant. Thus, the first trench 6 d thatis a through trench exposing the surface of the cap layer 2 e is formedin the protective insulating film 6. The first trench 6 d is formed suchthat its side wall surface is formed into an inclined surface by the wetetching, the width of a bottom portion is about 600 nm, and an upperportion is wider than the bottom portion. With the first trench 6 d, theprotective insulating film 6 progressively decreases in thickness fromthe second trench 6 c toward the first trench 6 d at a site between thefirst trench 6 d and the second trench 6 c. The first trench 6 d isformed at a formation scheduled site for a fine gate of a gate electrodeto be formed at subsequent processes, side by side with the secondtrench 6 c in the protective insulating film 6.

The resist mask 15 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 d after forming the second trench6 c in the protective insulating film 6 is exemplified in FIG. 17B andFIG. 17C, but the order of processes may be inverted so that the secondtrench 6 c may be formed after the first trench 6 d is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 18.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 d andis in Schottky contact with the surface of the compound semiconductorlayer 2. The over gate 7B fills the inside of the second trench 6 c andhas an OG end 7Ba located at a site away from an end portion on a drainelectrode 5 side of the second trench 6 c toward the drain electrode 5by about 0.1 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the Schottky-type AlGaN/GaN HEMT is formed.

Hereinafter, operations and effects that the Schottky-type AlGaN/GaNHEMT according to Modification Example 3 has will be described based oncomparison with a comparative example.

FIG. 19A and FIG. 19B are a view illustrating the Schottky-typeAlGaN/GaN HEMT according to Modification Example 3 of this embodimentand a chart presenting the intensity of an electric field applied to aregion between a source and a drain thereof. FIG. 19A is a schematiccross-sectional view of the AlGaN/GaN HEMT corresponding to FIG. 18, andFIG. 19B presents a characteristic chart of the electric fieldintensity. Note that the AlGaN/GaN HEMT in the comparative example isthe same as that in FIG. 4A, and its characteristic chart of theelectric field intensity is the same as that in FIG. 4B.

FIG. 19B presents the electric field intensity between a broken line Aand a broken line B drawn in FIG. 19A, and also presents electric fieldintensities of breakdown limits of device characteristics at an FG endand an OG end due to electric field concentration. The electric fieldintensity of breakdown limit at the FG end is defined as BE1, and theelectric field intensity of breakdown limit at the OG end is defined asBE2.

In the HEMT having the gate electrode in the overhanging shape, highelectric fields concentrate on the FG end and the OG end. In this case,the device characteristics are likely to deteriorate or break down moreat the FG end than at the OG end, so that BE1 is lower than BE2.

In the case of applying a predetermined drain voltage, the total amountof electric field generated around the gate electrode takes an almostconstant predetermined value. As described above, the deterioration orbreakdown of the device characteristics due to electric fieldconcentration most possibly occurs at the FG end. In contrast, anelectric field intensity close to the breakdown limit is not found in aregion between the FG end and the OG end. In Modification Example 3,focusing attention on this point, the electric field intensity in theregion between the FG end and the OG end is gradually increased to alimit not reaching the breakdown limit to relax the electric fieldconcentration on the FG end by the increase. In other words, a part ofthe electric field intensity at the FG end is distributed to the regionbetween the FG end and the OG end. This relaxes the electric fieldconcentration on the FG end which most possibly reaches the breakdownlimit, thereby suppressing the deterioration or breakdown of the devicecharacteristics due to the electric field concentration as a whole.

In the AlGaN/GaN HEMT according to Modification Example 3, the secondtrench 6 c is formed in the protective insulating film 6, and theinclined surface is formed between the first trench 6 d being a sitewhere the fine gate 7A is to be formed and the second trench 6 c. Thus,the protective insulating film 6 is reduced in thickness in the secondtrench 6 c and progressively reduced in thickness from the second trench6 c toward the first trench 6 d. The gate electrode 7 is formed so thatthe over gate 7B fills and contains an inclined surface portion of theprotective insulating film 6 and the second trench 6 c where theprotective insulating film 6 is thin. With this structure, a depletionlayer in the compound semiconductor layer 2 gradually extends. Asillustrated in FIG. 19B, the electric field intensity in the regionbetween the FG end 7Aa and the OG end 7Ba gradually increases to a limitnot reaching the breakdown limit and, along with this, the electricfield intensity at the FG end 7Aa decreases to relax the electric fieldconcentration. Thus, the electric field intensity at the FG end 7Aabecomes greatly lower than BE1 that is the breakdown limit. InModification Example 3, since a contributory portion of the increase inthe electric field intensity in the region between the FG end 7Aa andthe OG end 7Ba is larger than those in Modification Examples 1, 2, theelectric field intensity at the FG end 7Aa is lower than that inModification Example 2 and the electric field intensity at the OG end7Ba is lower than that in Modification Example 1. As described above,the deterioration or breakdown of the device characteristics due to theelectric field concentration is suppressed as a whole between the gateand the drain.

Three-terminal characteristics of the AlGaN/GaN HEMT according toModification Example 3 were investigated based on comparison with theabove comparative example. The results are presented in FIG. 20A andFIG. 20B. FIG. 20A presents the result of the comparative example, andFIG. 20B presents the result of Modification Example 3. Here, solidlines indicate IV characteristics at application of Vds=20 V, and brokenlines indicate IV characteristics at application of Vds=50 V.

In FIG. 20B, improvement in current collapse was confirmed as comparedto FIG. 20A. This means that the electric field concentration on the FGend was relaxed to suppress electron capture into an electron trap inModification Example 3.

Further, a high-temperature current conduction test was carried out onthe AlGaN/GaN HEMT according to Modification Example 3 based oncomparison with the above comparative example. The results are presentedin FIG. 21.

It was confirmed that, in Modification Example 3, the gate current lesschanged in the high-temperature current conduction test and no breakdownoccurred unlike the comparative example. In other words, application ofthe protective insulating film 6 and the gate electrode 7 inModification Example 3 realizes a highly reliable AlGaN/GaN HEMT withexcellent output characteristics.

As described above, according to Modification Example 3, a highlyreliable AlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower.

Second Embodiment

Hereinafter, a MIS-type AlGaN/GaN HEMT according to a second embodimentwill be described. This embodiment is different from the firstembodiment in that a gate insulating film is formed in the AlGaN/GaNHEMT of the first embodiment. Note that the same constituent members andso on as those of the AlGaN/GaN HEMT according to the first embodimentwill be denoted by the same reference signs, and a detailed descriptionthereof will be omitted.

FIG. 22A to FIG. 22C are schematic cross-sectional views illustratingmain processes in a method of manufacturing the MIS-type AlGaN/GaN HEMTaccording to the second embodiment.

First, through the processes in FIG. 1A to FIG. 2C of the firstembodiment, a first trench 6 a and a second trench 6 b are formed in aprotective insulating film 6 that covers the top of a compoundsemiconductor layer 2. The appearance in this event is illustrated inFIG. 22A.

Subsequently, as illustrated in FIG. 22B, a gate insulating film 21 thatcovers the inside of the first trench 6 a is formed.

More specifically, the gate insulating film 21 is formed on theprotective insulating film 6 in a manner to cover the inside of thefirst trench 6 a. For example, Al₂O₃ is deposited to a thickness ofabout (20) nm by an atomic layer deposition method, ALD method. Thus,the gate insulating film 21 is formed.

Incidentally, for the deposition of Al₂O₃, a plasma CVD method, asputtering method, or the like, for instance, may be used instead of theALD method. Further, instead of depositing Al₂O₃, a nitride or anoxynitride of Al may be used. Besides, an oxide, a nitride, anoxynitride of Si, Hf, Zr, Ti, Ta, or W or a multilayer of appropriatelyselected ones from among these may be deposited to form the gateinsulating film.

The gate insulating film 21 is formed on the protective insulating film6 and therefore formed to cover also the inside of the second trench 6b. Accordingly, the insulator is increased in thickness by the gateinsulating film 21 in the second trench 6 b. In this embodiment, takinginto consideration of this point, the thickness remaining at the bottomof the second trench 6 b is reduced in the process in FIG. 2B by theeffective thickness (the thickness converted into the protectiveinsulating film 6) expected thereafter in the formation of the gateinsulating film 21.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 22C.

A gate electrode 7 is in a so-called overhanging shape in which a finegate 7A at a lower part fills the inside of the first trench 6 d via thegate insulating film 21 and an over gate 7B at an upper part is formedwider than the fine gate 7A. In the gate electrode 7, one end (anelectrode end on a drain electrode 5 side, defined as an OG end 7Ba) ofthe over gate 75 is located inside the second trench 6 b via the gateinsulating film 21. Specifically, the OG end 7Ba is formed at a site,inside the second trench 6 b, away from an end portion on the gateelectrode 7 side of the second trench 6 b toward the drain electrode 5by 0.2 μm or more.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to this embodiment, a highly reliableAlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower, as in the first embodiment.

MODIFICATION EXAMPLES

Hereinafter, modification examples of the MIS-type AlGaN/GaN HEMTaccording to the second embodiment will be described.

Modification Example 1

Modification Example 1 has a structure in which a gate insulating filmis formed in the AlGaN/GaN HEMT according to Modification Example 1 ofthe first embodiment. Note that the same constituent members and so onas those of the AlGaN/GaN HEMT according to the first embodiment or thelike will be denoted by the same reference signs, and a detaileddescription thereof will be omitted.

FIG. 23A to FIG. 23C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 1 of the second embodiment.

First, through the processes in FIG. 1A to FIG. 2A and FIG. 8B to FIG.8C of the first embodiment, a first trench 6 a and a second trench 6 care formed in a protective insulating film 6 that covers the wholesurface of a compound semiconductor layer 2. The appearance in thisevent is illustrated in FIG. 23A.

Subsequently, as illustrated in FIG. 23B, a gate insulating film 22 thatcovers the inside of the first trench 6 a is formed.

More specifically, the gate insulating film 22 is formed on theprotective insulating film 6 in a manner to cover the inside of thefirst trench 6 a. For example, Al₂O₃ is deposited to a thickness ofabout 20 nm by an ALD method. Thus, the gate insulating film 22 isformed.

Incidentally, for the deposition of Al₂O₃, a plasma CVD method, asputtering method, or the like, for instance, may be used instead of theALD method. Further, instead of depositing Al₂O₃, a nitride or anoxynitride of Al may be used. Besides, an oxide, a nitride, anoxynitride of Si, Hf, Zr, Ti, Ta, or W or a multilayer of appropriatelyselected ones from among these may be deposited to form the gateinsulating film.

The gate insulating film 22 is formed on the protective insulating film6 and therefore formed to cover also the inside of the second trench 6c. Accordingly, the insulator is increased in thickness by the gateinsulating film 22 in the second trench 6 c. In this embodiment, takinginto consideration of this point, the thickness remaining at the bottomof the second trench 6 c is reduced in the process in FIG. 8B by theeffective thickness (the thickness converted into the protectiveinsulating film 6) expected thereafter in the formation of the gateinsulating film 22.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 23C.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 a viathe gate insulating film 22. The over gate 7B fills the inside of thesecond trench 6 c via the gate insulating film 22 and has an OG end 7Balocated at a site away from an end portion on a drain electrode 5 sideof the second trench 6 c toward the drain electrode 5 by about 0.1 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to Modification Example 1, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 1 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power.

Modification Example 2

Modification Example 2 has a structure in which a gate insulating filmis formed in the AlGaN/GaN HEMT according to Modification Example 2 ofthe first embodiment. Note that the same constituent members and so onas those of the AlGaN/GaN HEMT according to the first embodiment or thelike will be denoted by the same reference signs, and a detaileddescription thereof will be omitted.

FIG. 24A to FIG. 24C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 2 of the second embodiment.

First, through the processes in FIG. 1A to FIG. 2B of the firstembodiment and FIG. 13B in Modification Example 2 of the firstembodiment, a first trench 6 d and a second trench 6 b are formed in aprotective insulating film 6 that covers the whole surface of a compoundsemiconductor layer 2. The appearance in this event is illustrated inFIG. 24A.

Subsequently, as illustrated in FIG. 24B, a gate insulating film 23 thatcovers the inside of the first trench 6 d is formed.

More specifically, the gate insulating film 23 is formed on theprotective insulating film 6 in a manner to cover the inside of thefirst trench 6 d. For example, Al₂O₃ is deposited to a thickness ofabout 20 nm by an atomic layer deposition method, ALD method. Thus, thegate insulating film 23 is formed.

Incidentally, for the deposition of Al₂O₃, a plasma CVD method, asputtering method, or the like, for instance, may be used instead of theALD method. Further, instead of depositing Al₂O₃, a nitride or anoxynitride of Al may be used. Besides, an oxide, a nitride, anoxynitride of Si, Hf, Zr, Ti, Ta, or W or a multilayer of appropriatelyselected ones from among these may be deposited to form the gateinsulating film.

The gate insulating film 23 is formed on the protective insulating film6 and therefore formed to cover also the inside of the second trench 6b. Accordingly, the insulator is increased in thickness by the gateinsulating film 23 in the second trench 6 b. In this embodiment, takinginto consideration of this point, the thickness remaining at the bottomof the second trench 6 b is reduced in the process in FIG. 2B by theeffective thickness (the thickness converted into the protectiveinsulating film 6) expected thereafter in the formation of the gateinsulating film 23.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 24C.

A gate electrode 7 is in a so-called overhanging shape in which a finegate 7A at a lower part fills the inside of the first trench 6 d via thegate insulating film 23 and an over gate 7B at an upper part is formedwider than the fine gate 7A. In the gate electrode 7, one end (anelectrode end on a drain electrode 5 side, defined as an OG end 7Ba) ofthe over gate 7B is located inside the second trench 6 b via the gateinsulating film 23. Specifically, the OG end 7Ba is formed at a site,inside the second trench 6 b, away from an end portion on the gateelectrode 7 side of the second trench 6 b toward the drain electrode 5by 0.2 μm or more, here about 0.2 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to Modification Example 2, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 2 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power.

Modification Example 3

Modification Example 3 has a structure in which a gate insulating filmis formed in the AlGaN/GaN HEMT according to Modification Example 3 ofthe first embodiment. Note that the same constituent members and so onas those of the AlGaN/GaN HEMT according to the first embodiment or thelike will be denoted by the same reference signs, and a detaileddescription thereof will be omitted.

FIG. 25A to FIG. 25C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 3 of the second embodiment.

First, through the processes in FIG. 1A to FIG. 2A of the firstembodiment and FIG. 17B to FIG. 17C in Modification Example 3 of thefirst embodiment, a first trench 6 d and a second trench 6 c are formedin a protective insulating film 6 that covers the whole surface of acompound semiconductor layer 2. The appearance in this event isillustrated in FIG. 25A.

Subsequently, as illustrated in FIG. 25B, a gate insulating film 24 thatcovers the inside of the first trench 6 d is formed.

More specifically, the gate insulating film 24 is formed on theprotective insulating film 6 in a manner to cover the inside of thefirst trench 6 d. For example, Al₂O₃ is deposited to a thickness ofabout 20 nm by an atomic layer deposition method, ALD method. Thus, thegate insulating film 24 is formed.

Incidentally, for the deposition of Al₂O₃, a plasma CVD method, asputtering method, or the like, for instance, may be used instead of theALD method. Further, instead of depositing Al₂O₃, a nitride or anoxynitride of Al may be used. Besides, an oxide, a nitride, anoxynitride of Si, Hf, Zr, Ti, Ta, or W or a multilayer of appropriatelyselected ones from among these may be deposited to form the gateinsulating film.

The gate insulating film 24 is formed on the protective insulating film6 and therefore formed to cover also the inside of the second trench 6c. Accordingly, the insulator is increased in thickness by the gateinsulating film 24 in the second trench 6 c. In this embodiment, takinginto consideration of this point, the thickness remaining at the bottomof the second trench 6 c is reduced in the process in FIG. 17B by theeffective thickness (the thickness converted into the protectiveinsulating film 6) expected thereafter in the formation of the gateinsulating film 24.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 25C.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 d viathe gate insulating film 24. The over gate 7B fills the inside of thesecond trench 6 c via the gate insulating film 24 and has an OG end 7Balocated at a site away from an end portion on a drain electrode 5 sideof the second trench 6 c toward the drain electrode 5 by about 0.1 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMP is formed.

As described above, according to Modification Example 3, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 3 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power.

Third Embodiment

Hereinafter, a MIS-type AlGaN/GaN HEMT according to a third embodimentwill be described. This embodiment is different from the firstembodiment in that a gate insulating film is formed in the AlGaN/GaNHEMT of the first embodiment. Note that the same constituent members andso on as those of the AlGaN/GaN HEMT according to the first embodimentwill be denoted by the same reference signs, and a detailed descriptionthereof will be omitted.

FIG. 26A to FIG. 26C are schematic cross-sectional views illustratingmain processes in a method of manufacturing the MIS-type AlGaN/GaN HEMTaccording to the third embodiment.

First, through the processes in FIG. 1A to FIG. 2B of the firstembodiment, a second trench 6 b is formed in a protective insulatingfilm 6 that covers the top of a compound semiconductor layer 2. Theappearance in this event is illustrated in FIG. 26A.

Subsequently, as illustrated in FIG. 26B, a first trench 6 e is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 25having an opening 25 a is formed.

Next, dry etching using the resist mask 25 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness at the bottom of the opening25 a. The remaining portion of the protective insulating film 6 servesas a gate insulating film, and therefore the predetermined thickness isset to, for example, about 20 nm. For example, SF₆ is used as an etchinggas. Thus, the first trench 6 e having a width of about 600 nm and adepth of, for example, about 40 nm is formed in the protectiveinsulating film 6. The first trench 6 e is formed at a formationscheduled site for a fine gate of a gate electrode to be formed atsubsequent processes, side by side with the second trench 6 b in theprotective insulating film 6.

The resist mask 25 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 e after forming the second trench6 b in the protective insulating film 6 is exemplified in FIG. 2B andFIG. 26B, but the order of processes may be inverted so that the secondtrench 6 b may be formed after the first trench 6 e is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 26C.

A gate electrode 7 is in a so-called overhanging shape in which a finegate 7A at a lower part fills the inside of the first trench 6 e and anover gate 7B at an upper part is formed wider than the fine gate 7A. Inthe gate electrode 7, the fine gate 7A is located via a cap layer 2 eand the protective insulating film 6 at the bottom of the first trench 6e, and one end (an electrode end on a drain electrode 5 side, defined asan OG end 7Ba) of the over gate 7B is located inside the second trench 6b. Specifically, the OG end 7Ba is formed at a site, inside the secondtrench 6 b, away from an end portion on the gate electrode 7 side of thesecond trench 6 b toward the drain electrode 5 by 0.2 μm or more, hereabout 0.2 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to this embodiment, a highly reliableAlGaN/GaN HEMT is realized which relaxes the electric fieldconcentration around the gate electrode 7 by a relatively simplestructure to suppress deterioration or breakdown of the devicecharacteristics so as to achieve high withstand voltage and high outputpower, as in the first embodiment. Further, since the part of theprotective insulating film 6 also serves as the gate insulating filmwhen forming the gate insulating film forming the MIS type,manufacturing processes are reduced.

MODIFICATION EXAMPLES

Hereinafter, modification examples of the MIS-type AlGaN/GaN HEMTaccording to the third embodiment will be described.

Modification Example 1

Modification Example 1 has a structure in which a part of the protectiveinsulating film also serves as the gate insulating film in the AlGaN/GaNHEMT according to Modification Example 1 of the first embodiment. Notethat the same constituent members and so on as those of the AlGaN/GaNHEMT according to the first embodiment or the like will be denoted bythe same reference signs, and a detailed description thereof will beomitted.

FIG. 27A to FIG. 27C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 1 of the third embodiment.

First, through the processes in FIG. 1A to FIG. 2A of the firstembodiment and FIG. 8B in Modification Example 1 of the firstembodiment, a second trench 6 c is formed in a protective insulatingfilm 6 that covers the whole surface of a compound semiconductor layer2. The appearance in this event is illustrated in FIG. 27A.

Subsequently, as illustrated in FIG. 27B, a first trench 6 e is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 25having an opening 25 a is formed.

Next, dry etching using the resist mask 25 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness in the opening 25 a. Theremaining portion of the protective insulating film 6 serves as a gateinsulating film, and therefore the predetermined thickness is set to,for example, about 20 nm. For example, SF₆ is used as an etching gas.Thus, the first trench 6 e having a width of about 600 nm and a depthof, for example, about 40 nm is formed in the protective insulating film6. The first trench 6 e is formed at a formation scheduled site for afine gate of a gate electrode to be formed at subsequent processes, sideby side with the second trench 6 c in the protective insulating film 6.

The resist mask 25 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 e after forming the second trench6 c in the protective insulating film 6 is exemplified in FIG. 8B andFIG. 27B, but the order of processes may be inverted so that the secondtrench 6 c may be formed after the first trench 6 e is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 27C.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 e via acap layer 2 e and the protective insulating film 6 at the bottom of thefirst trench 6 e. The over gate 7B fills the inside of the second trench6 c and has an OG end 7Ba located at a site away from an end portion ona drain electrode 5 side of the second trench 6 c toward the drainelectrode 5 by about 0.1 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to Modification Example 1, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 1 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power. Further, sincethe part of the protective insulating film 6 also serves as the gateinsulating film when forming the gate insulating film forming the MIStype, manufacturing processes are reduced.

Modification Example 2

Modification Example 2 has a structure in which a part of the protectiveinsulating film also serves as the gate insulating film in the AlGaN/GaNHEMT according to Modification Example 2 of the first embodiment. Notethat the same constituent members and so on as those of the AlGaN/GaNHEMT according to the first embodiment or the like will be denoted bythe same reference signs, and a detailed description thereof will beomitted.

FIG. 28A to FIG. 28C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 2 of the third embodiment.

First, through the processes in FIG. 1A to FIG. 2B of the firstembodiment, a second trench 6 b is formed in a protective insulatingfilm 6 that covers the whole surface of a compound semiconductor layer2. The appearance in this event is illustrated in FIG. 28A.

Subsequently, as illustrated in FIG. 28B, a first trench 6 f is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 26having an opening 26 a is formed.

Next, wet etching using the resist mask 26 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness at the bottom of the opening26 a. The remaining portion of the protective insulating film 6 servesas a gate insulating film, and therefore the predetermined thickness isset to, for example, about 20 nm. For example, buffered hydrofluoricacid is used as an etchant. Thus, the first trench 6 f is formed in theprotective insulating film 6. The first trench 6 f is formed such thatthe depth is, for example, 40 nm, its side wall surface is formed intoan inclined surface by the wet etching, the width of a bottom portion isabout 600 nm, and an upper portion is wider than the bottom portion.With the first trench 6 f, the protective insulating film 6progressively decreases in thickness from the second trench 6 b towardthe first trench 6 f at a site between the first trench 6 f and thesecond trench 6 b. The first trench 6 f is formed at a formationscheduled site for a fine gate of a gate electrode to be formed atsubsequent processes, side by side with the second trench 6 b in theprotective insulating film 6.

The resist mask 26 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 f after forming the second trench6 b in the protective insulating film 6 is exemplified in FIG. 2B andFIG. 28B, but the order of processes may be inverted so that the secondtrench 6 b may be formed after the first trench 6 f is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 28C.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 f via acap layer 2 e and the protective insulating film 6 at the bottom of thefirst trench 6 f. The over gate 7B has one end (an electrode end on adrain electrode 5 side, defined as an OG end 7Ba) located inside thesecond trench 6 b. Specifically, the OG end 7Ba is formed at a site,inside the second trench 6 b, away from an end portion on the gateelectrode 7 side of the second trench 6 b toward the drain electrode 5by 0.2 μm or more, here about 0.2 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to Modification Example 2, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 2 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power. Further, sincethe part of the protective insulating film 6 also serves as the gateinsulating film when forming the gate insulating film forming the MIStype, manufacturing processes are reduced.

Modification Example 3

Modification Example 3 has a structure in which a part of the protectiveinsulating film also serves as the gate insulating film in the AlGaN/GaNHEMT according to Modification Example 3 of the first embodiment. Notethat the same constituent members and so on as those of the AlGaN/GaNHEMT according to the first embodiment or the like will be denoted bythe same reference signs, and a detailed description thereof will beomitted.

FIG. 29A to FIG. 29C are schematic cross-sectional views illustratingmain processes in a method of manufacturing a MIS-type AlGaN/GaN HEMTaccording to Modification Example 3 of the third embodiment.

First, through the processes in FIG. 1A to FIG. 2A of the firstembodiment and FIG. 17B in Modification Example 3 of the firstembodiment, a second trench 6 c is formed in a protective insulatingfilm 6 that covers the whole surface of a compound semiconductor layer2. The appearance in this event is illustrated in FIG. 29A.

Subsequently, as illustrated in FIG. 29B, a first trench 6 f is formedin the protective insulating film 6.

More specifically, a resist is first applied on the whole surface of theprotective insulating film 6. For example, PFI-32 (trade name)manufactured by Sumitomo Chemical Co., Ltd. is used as the resist. Anultraviolet method is used to perform, for example, exposure for anopening having a width of 600 nm on the applied resist, and the resistis developed. For example, NMD-W (trade name) manufactured by Tokyo OhkaKogyo Co., Ltd. is used as a developing solution. Thus, a resist mask 26having an opening 26 a is formed.

Next, wet etching using the resist mask 26 is performed on theprotective insulating film 6 so that the protective insulating film 6remains with only a predetermined thickness at the bottom of the opening26 a. The remaining portion of the protective insulating film 6 servesas a gate insulating film, and therefore the predetermined thickness isset to, for example, about 20 nm. For example, buffered hydrofluoricacid is used as an etchant. Thus, the first trench 6 f is formed in theprotective insulating film 6. The first trench 6 f is formed such thatthe depth is, for example, 40 nm, its side wall surface is formed intoan inclined surface by the wet etching, the width of the bottom portionis about 600 nm, and an upper portion is wider than a bottom portion.With the first trench 6 f, the protective insulating film 6progressively decreases in thickness from the second trench 6 c towardthe first trench 6 f at a site between the first trench 6 f and thesecond trench 6 c. The first trench 6 f is formed at a formationscheduled site for a fine gate of a gate electrode to be formed atsubsequent processes, side by side with the second trench 6 c in theprotective insulating film 6.

The resist mask 26 is removed by ashing using oxygen plasma or wettreatment using a chemical.

The case of forming the first trench 6 f after forming the second trench6 c in the protective insulating film 6 is exemplified in FIG. 17B andFIG. 29B, but the order of processes may be inverted so that the secondtrench 6 c may be formed after the first trench 6 f is formed in theprotective insulating film 6.

Subsequently, the processes in FIG. 3A to FIG. 3C of the firstembodiment are performed. The state corresponding to FIG. 3C isillustrated in FIG. 29C.

A gate electrode 7 is in an overhanging shape composed of a fine gate 7Aat a lower part and an over gate 7B at an upper part wider than the finegate 7A. The fine gate 7A fills the inside of the first trench 6 f via acap layer 2 e and the protective insulating film 6 at the bottom of thefirst trench 6 f. The over gate 7B fills the inside of the second trench6 c and has an OG end 7Ba located at a site away from an end portion ona drain electrode 5 side of the second trench 6 c toward the drainelectrode 5 by about 0.1 μm.

Thereafter, through processes of electrical connection of a sourceelectrode 4, the drain electrode 5, and the gate electrode 7 and so on,the MIS-type AlGaN/GaN HEMT is formed.

As described above, according to Modification Example 3, the electricfield concentration around the gate electrode 7 is relaxed by arelatively simple structure to suppress deterioration or breakdown ofthe device characteristics as in Modification Example 3 of the firstembodiment. This realizes a highly reliable AlGaN/GaN HEMT whichachieves high withstand voltage and high output power. Further, sincethe part of the protective insulating film 6 also serves as the gateinsulating film when forming the gate insulating film forming the MIStype, manufacturing processes are reduced.

Fourth Embodiment

This embodiment discloses a power supply device including one kindselected from among the AlGaN/GaN HEMTs according to the first to thirdembodiments and their modification examples.

FIG. 30 is a connection diagram illustrating a schematic configurationof a power supply device according to a fourth embodiment.

The power supply device according to this embodiment includes ahigh-voltage primary-side circuit 31, a low-voltage secondary-sidecircuit 32, and a transformer 33 disposed between the primary-sidecircuit 31 and the secondary-side circuit 32.

The primary-side circuit 31 includes an AC power supply 34, a so-calledbridge rectifying circuit 35, and a plurality of (four here) switchingelements 36 a, 36 b, 36 c, 36 d. Further, the bridge rectifying circuit35 has a switching element 36 e.

The secondary-side circuit 32 includes a plurality of (three here)switching elements 37 a, 37 b, 37 c.

In this embodiment, the switching elements 36 a, 36 b, 36 c, 36 d, 36 eof the primary-side circuit 31 are each one kind selected from among theAlGaN/GaN HEMTs according to the first to third embodiments and theirmodification examples. On the other hand, the switching elements 37 a,37 b, 37 c of the secondary-side circuit 32 are each an ordinary MIS•FETusing silicon.

In this embodiment, the AlGaN/GaN HEMT that relaxes the electric fieldconcentration around a gate electrode 7 by a relatively simple structureto suppress deterioration or breakdown of the device characteristics isapplied to the high-voltage circuit. This realizes a highly reliablelarge-power power supply circuit.

Fifth Embodiment

This embodiment discloses a high-frequency amplifier including one kindselected from among the AlGaN/GaN HEMTs according to the first to thirdembodiments and their modification examples.

FIG. 31 is a connection diagram illustrating a schematic configurationof a high-frequency amplifier according to a fifth embodiment.

The high-frequency amplifier according to this embodiment includes adigital pre-distortion circuit 41, mixers 42 a, 42 b, and a poweramplifier 43.

The digital pre-distortion circuit 41 compensates nonlinear distortionof an input signal. The mixer 42 a mixes the input signal whosenonlinear distortion is compensated and an AC signal. The poweramplifier 43 amplifies the input signal mixed with the AC signal, andhas one kind selected from among the AlGaN/GaN HEMTs according to thefirst to third embodiments and their modification examples. In FIG. 31,by, for example, changing the switches, an output-side signal can bemixed with the AC signal by the mixer 42 b, and the resultant can besent out to the digital pre-distortion circuit 41.

In this embodiment, the AlGaN/GaN HEMT that relaxes the electric fieldconcentration around a gate electrode 7 by a relatively simple structureto suppress deterioration or breakdown of the device characteristics isapplied to the high-frequency amplifier. This realizes a highly reliablehigh-withstand-voltage high-frequency amplifier.

Other Embodiments

In the first to third embodiments and their modification examples, andthe fourth and fifth embodiments, the AlGaN/GaN HEMTs are exemplified asthe compound semiconductor devices. Other than the AlGaN/GaN HEMTs, thefollowing HEMTs are applicable as the compound semiconductor devices.

Other HEMT Example 1

This example discloses an InAlN/GaN HEMT as a compound semiconductordevice.

InAlN and GaN are compound semiconductors whose lattice constants can bemade close to each other by their compositions. In this case, in theabove-described first to third embodiments and their modificationexamples and the fourth to fifth embodiments, the electron transit layeris formed of i-GaN, the intermediate layer is formed of AlN, theelectron supply layer is formed of n-InAlN, and the cap layer is formedof n-GaN. The n-GaN of the cap layer can be omitted as necessary.Further, since almost no piezoelectric polarization occurs in this case,two-dimensional electron gas is mainly generated by spontaneouspolarization of InAlN.

According to this example, the electric field concentration around thegate electrode is relaxed by a relatively simple structure to suppressdeterioration or breakdown of the device characteristics as in theabove-described AlGaN/GaN HEMTs. This realizes a highly reliableInAlN/GaN HEMT which achieves high withstand voltage and high outputpower.

Other HEMT Example 2

This example discloses an InAlGaN/GaN HEMT as a compound semiconductordevice.

GaN and InAlGaN are compound semiconductors that the lattice constant ofthe latter is smaller than the lattice constant of the former. In thiscase, in the above-described first to third embodiments and theirmodification examples and the fourth to fifth embodiments, the electrontransit layer is formed of i-GaN, the intermediate layer is formed ofi-InAlGaN, the electron supply layer is formed of n-InAlGaN, and the caplayer is formed of n⁺-GaN. The n⁺-GaN of the cap layer can be omitted asnecessary.

According to this example, the electric field concentration around thegate electrode is relaxed by a relatively simple structure to suppressdeterioration or breakdown of the device characteristics as in theabove-described AlGaN/GaN HEMTs. This realizes a highly reliableInAlGaN/GaN HEMT which achieves high withstand voltage and high outputpower.

According to the above-described aspects, a highly reliable compoundsemiconductor device can be realized which relaxes the electric fieldconcentration around an electrode by a relatively simple structure tosuppress deterioration or breakdown of device characteristics so as toachieve high withstand voltage and high output power.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A compound semiconductor device, comprising: acompound semiconductor layer; a protective insulating film that covers atop of the compound semiconductor layer; and a gate electrode in which afirst portion at a lower part and a second portion being wider than thefirst portion at an upper part are integrally formed, and a lowersurface of the first portion is formed on the protective insulating filmor on the compound semiconductor layer, wherein the protectiveinsulating film has a first trench and a non-through second trench whichis formed side by side with the first trench, and wherein the firstportion of the gate electrode fills the first trench, the second portioncovers a part of an upper surface of the protective insulating film, andone end of the second portion is away from the first trench and locatedat least in the second trench.
 2. The compound semiconductor deviceaccording to claim 1, wherein the protective insulating film decreasesin thickness at a site between the first trench and the second trenchprogressively from the second trench toward the first trench.
 3. Thecompound semiconductor device according to claim 1, wherein theelectrode fills the second trench, and the one end is located beyond thesecond trench in a direction away from the first trench.
 4. The compoundsemiconductor device according to claim 1, wherein the gate electrodefills the first trench formed as a through trench and is in contact withthe compound semiconductor layer.
 5. The compound semiconductor deviceaccording to claim 1, wherein the first trench is formed such that theprotective insulating film remains with only a predetermined thicknesson a bottom thereof, and wherein the electrode is formed on the compoundsemiconductor layer via a bottom portion of the protective insulatingfilm.
 6. The compound semiconductor device according to claim 1, whereinthe first trench is formed as a through trench, wherein the compoundsemiconductor device further comprises a gate insulating film formed onthe protective insulating film in a manner to cover an inner wallsurface of the first trench, and wherein the electrode is formed on thecompound semiconductor layer via the gate insulating film.
 7. A powersupply circuit comprising a transformer, and a high-voltage circuit anda low-voltage circuit across the transformer, the high-voltage circuitcomprising a transistor, the transistor comprising: a compoundsemiconductor layer; a protective insulating film that covers a top ofthe compound semiconductor layer; and a gate electrode in which a firstportion at a lower part and a second portion being wider than the firstportion at an upper part are integrally formed, and a lower surface ofthe first portion is formed on the protective insulating film or on thecompound semiconductor layer, wherein the protective insulating film hasa first trench and a non-through second trench which is formed side byside with the first trench, wherein the first portion of the gateelectrode fills the first trench, the second portion covers a part of anupper surface of the protective insulating film, and one end of theelectrode second portion is away from the first trench and located atleast in the second trench.